Method for iii-v/silicon hybrid integration

ABSTRACT

A method of transfer printing. The method comprising: providing a precursor photonic device, comprising a substrate and a bonding region, wherein the precursor photonic device includes one or more alignment marks located in or adjacent to the bonding region; providing a transfer die, said transfer die including one or more alignment marks; aligning the one or more alignment marks of the precursor photonic device with the one or more alignment marks of the transfer die; and bonding at least a part of the transfer die to the bonding region.

The present application is a continuation of U.S. patent application Ser. No. 17/167,065, filed Feb. 3, 2021, entitled “METHOD FOR III-V/SILICON HYBRID INTEGRATION”, which is a continuation-in-part of U.S. patent application Ser. No. 16/532,406, filed Aug. 5, 2019, entitled “METHOD FOR III-V/SILICON HYBRID INTEGRATION”, which (i) claims priority to and the benefit of U.S. Provisional Application No. 62/715,123, filed Aug. 6, 2018, entitled “METHOD FOR III-V/SI HYBRID INTEGRATION BY MICRO-TRANSFER PRINTING PROCESS”, and (ii) claims priority to and the benefit of U.S. Provisional Application No. 62/880,585, filed Jul. 30, 2019, entitled “METHOD FOR III-V/SILICON HYBRID INTEGRATION”; the entire contents of all of the documents identified in this paragraph are incorporated herein by reference.

The present invention relates to a precursor photonic device, a transfer die, a platform wafer, a method of preparing a precursor photonic device, a method of forming a transfer die, and a method of transfer printing.

Micro-transfer printing (MTP) can be used to make hybrid III-V/Si integrated photonic components and circuits, for example as discussed in LOI et al, Transfer Printing of AlGalnAs/InP Etched Facet Lasers to Si Substrates, IEEE Photonics Journal, Vol. 8, No. 6, December 2016 the entire contents of which is incorporated herein by reference. Broadly, MTP involves the creation of a pre-cursor device on a first substrate. The pre-cursor device is then lifted, e.g. via an elastomer stamp, from the first substrate and deposited onto a second substrate.

However, conventional MTP techniques suffer from three main issues: (i) low alignment accuracy (±0.5-1.5 μm) between the III-V device and Si platform; (ii) low III-V/Si bonding adhesion, which leads to low reliability and can require the use of extra materials such as solder to increase adhesion strength; and (iii) low throughput, as many hybrid integration schemes require bonding of individual III-V dies or patches of material to a Si platform, one at a time.

It would be advantageous then to both increase the alignment between the III-V device and Si platform (so as to decrease optical losses in the system), as well as to increase the throughput of hybrid integration schemes.

At a general level, embodiments of the present invention provide III-V devices or precursor devices, as well as Si platforms, which include alignment marks to aid in a method of transfer printing.

In a first aspect, embodiments of the invention provide a method of transfer printing, comprising:

-   -   providing a precursor photonic device, comprising a substrate         and a bonding region, wherein the precursor photonic device         includes one or more alignment marks located in or adjacent to         the bonding region;     -   providing a transfer die, said transfer die including one or         more alignment marks;     -   aligning the one or more alignment marks of the precursor         photonic device with the one or more alignment marks of the         transfer die; and     -   bonding at least a part of the transfer die to the bonding         region.

The method according to the first aspect can advantageously result in alignment accuracies within ±200 nm. Typically the alignment accuracy is influenced by factors including, in the x and y directions: alignment mark design and fabrication; microscopic magnification; resolution of translation stage; and alignment operation, and in the z direction: the accuracy with which the bonding region is fabricated (e.g. how accurately the cavity is etched); the accuracy with which the transfer die is fabricated (e.g. how accuracy the epitaxial growth of the layers is performed; and the alignment operation.

The method may have any one or, to the extent that they are compatible, any combination of the following optional features.

The precursor photonic device, or the transfer die, or both, may include one or more metal patches located at least adjacent to the respective alignment marks. The one or more alignment marks located in the or adjacent to the bonding region or the at least one or more alignment marks included in the transfer die, or both, may include one or more trenches surrounding the alignment marks, which may have been patterned and etched at the same time as the waveguides. Both the one or more alignment marks and one or more trenches are covered by the covered by the metal patch.

The precursor photonic device may include a first waveguide, and the transfer die may include a second waveguide and, once bonded, an interface between the first waveguide and the second waveguide may be angled relative to the guiding direction of the first waveguide or second waveguide. The first waveguide, the second waveguide, or both, may include a T-bar facet.

At least one of the alignment marks located in or adjacent to the bonding region or at least one of the one or more alignment marks included in the transfer die, or both, may be formed in one of the following shapes: a rectangle, a square, an L-shape, a circle, a ring, a partial circle, and a partial ring.

The precursor photonic device may include one or more alignment marks located adjacent to the bonding region, and once bonded, a closest distance between one of the one or more alignment marks adjacent to the bonding region and a corresponding one of the one or more alignment marks included in the transfer die may be at least 50 μm and no more than 1000 μm.

The or each alignment mark located in or adjacent to the bonding region may be symmetrical as compared to a corresponding alignment mark included in the transfer die.

The precursor photonic device may have any, or any combination insofar as they are compatible, of the optional features of the precursor photonic device of the second aspect.

The transfer die may have any, or any combination insofar as they are compatible, of the optional features of the transfer die of the third aspect.

The method may include a step of filling a facet between the precursor photonic device and the transfer die. The filling material used to fill the facet may be silicon nitride, amorphous silicon, a dielectric, or a benzocyclobutene resin.

The method may comprise one or more steps of: plasma treating the precursor photonic device and/or the transfer die; dipping the precursor photonic device in water; drying the precursor photonic device; and annealing the transfer die and precursor photonic device. The annealing may be performed at a temperature of at least 250° C. and no more than 350° C. for a time of at least 20 minutes and no more than 40 minutes or no more than 400 minutes. The annealing step may be performed in an inert gas atmosphere, such as a nitrogen atmosphere or argon atmosphere.

These parameters used in the transfer printing process have been found to increase the bond adhesion and so increase the yield.

In a second aspect, embodiments of the present invention provide an optoelectronic device, produced using the method of the first aspect, including any one, or any combination insofar as they are compatible, of the optional features discussed with relation thereto.

In a third aspect, embodiments of the present invention provide an optoelectronic device comprising:

-   -   a silicon-on-insulator wafer, having a cavity; and     -   a III-V semiconductor based photonic device, located within and         bonded to the cavity;     -   wherein the III-V semiconductor based photonic device includes         one or more alignment marks, which align with corresponding         alignment mark(s) on the silicon-on-insulator wafer.

Advantageously, such an optoelectronic device incurs less loss as a signal propagates through it due to the increased accuracy in alignment between the III-V semiconductor based photonic device and other components of the optoelectronic device.

The alignment marks in the III-V semiconductor based device may overlap with the corresponding alignment marks on the silicon-on-insulator wafer, which may be located in the cavity.

The optoelectronic device of the third aspect may have any one, or any combination insofar as they are compatible, of the optional features set out below.

The one or more alignment marks may be located in an optically transparent region of the III-V semiconductor based photonic device.

The one or more alignment marks may be voids in the III-V semiconductor based photonic device that extend entirely through the III-V semiconductor based photonic device.

The optoelectronic device may include an input and/or output waveguide, the waveguide(s) being provided in the silicon-on-insulator wafer and optically coupled to the III-V semiconductor based photonic device.

The silicon-on-insulator wafer, or the III-V semiconductor based device, or both, may include one or more metal patches located at least adjacent to the respective alignment marks. The one or more alignment marks located on the silicon-on-insulator wafer or the at least one or more alignment included in the III-V semiconductor based device, or both, may include one or more trenches surrounding the alignment marks, which may be patterned and etched at the same time as the waveguides. Both the one or more alignment marks and one or more trenches are covered by the metal patch.

The silicon-on-insulator wafer may include a first waveguide, and the III-V semiconductor based device may include a second waveguide and, once bonded, an interface between the first waveguide and the second waveguide may be angled relative to the guiding direction of the first waveguide or second waveguide. The first waveguide, the second waveguide, or both, may include a T-bar facet.

At least one of the alignment marks located on the silicon-on-insulator wafer or at least one of the one or more alignment marks included in the III-V semiconductor based device, or both, may be formed in one of the following shapes: a rectangle, a square, an L-shape, a circle, a ring, a partial circle, and a partial ring.

The silicon-on-insulator wafer may include one or more alignment marks located adjacent to the bonding region, and once bonded, a closest distance between one of the one or more alignment marks adjacent to the bonding region and a corresponding one of the one or more alignment marks included in the III-V semiconductor based device may be at least 50 μm and no more than 1000 μm.

The or each alignment mark located in or adjacent to the bonding region may be symmetrical as compared to a corresponding alignment mark included in the transfer die.

In a fourth aspect, embodiments of the present invention provide an optoelectronic device comprising:

-   -   a precursor photonic device according to the fifth aspect,         bonded to a transfer die according to the sixth aspect.

The precursor photonic device may have any one, or any combination of, the optional features of the precursor photonic device of the fifth aspect. The transfer die may have any one, or any combination of, the optional features of the transfer die of the sixth aspect.

In a fifth aspect, embodiments of the present invention provide a precursor photonic device, comprising:

-   -   a substrate;     -   a bonding region, for receiving and bonding to a transfer die;         and     -   one or more alignment marks, for use in transfer printing, said         alignment marks being located in or adjacent to the bonding         region.

Such a precursor device can result in a photonic device with increased alignment between one or more optical components once bonded to the bonding region, which can decrease undesirable losses as light passes through the photonic device.

The precursor photonic device may have any one or, to the extent that they are compatible, any combination of the following optional features.

The precursor photonic device may include one or more metal patches located at least adjacent to the respective alignment marks. The one or more alignment marks located in the or adjacent to the bonding region may include one or more trenches surrounding the alignment marks, which may be patterned and etched at the same time as the waveguides. Both the one or more alignment marks and one or more trenches are covered by the metal patch.

The precursor photonic device may include a first waveguide and an interface thereof may be angled relative to the guiding direction of the first waveguide. The first waveguide may include a T-bar facet.

At least one of the alignment marks located in or adjacent to the bonding region may be formed in one of the following shapes: a rectangle, a square, an L-shape, a circle, a ring, a partial circle, and a partial ring.

The bonding region may be a cavity, provided in the substrate. The alignment marks may be etched in the cavity. The alignment marks may, alternatively, be etched in a region of the precursor device proximal to but not within the cavity. This can allow the alignment marks to be self-aligned with any optical components existing in the precursor device before bonding.

The alignment marks may be located either in the bonding region, or in a region of the precursor device proximal to the bonding region but not within it. In examples where the alignment marks are in a region of the precursor device proximal to the bonding region but not within it, this can increase the adhesion strength between the precursor device and a subsequently transferred photonic device.

The precursor photonic device may further comprise an input waveguide, wherein the alignment marks are configured to align a photonic device, located on the transfer die, relative to the input waveguide.

The one or more alignment marks may allow for alignment in at least two non-parallel directions.

The one or more alignment marks may be provided as one or more etched regions and/or one or more patterned metal surfaces. The etched regions may have a depth of at least 100 nm and no more than 3000 nm.

The precursor photonic device may include one or more coarse alignment marks, and one or more fine alignment marks. The one or more coarse alignment marks may project in at least two non-parallel directions. The one or more coarse alignment marks may be any one or more of: an arrow, a cross, a T shape, and an L shape. There may be two or more fine alignment marks which respectively project in at least two non-parallel directions. The one or more fine alignment mark(s) may include Vernier patterns.

The precursor photonic device may be a silicon-on-insulator wafer, including either or both of an input waveguide and an output waveguide, each adjacent to the bonding region.

In a second sixth aspect, embodiments of the invention provide a transfer die comprising:

-   -   a photonic device, said photonic device having a bonding surface         suitable for bonding to a precursor photonic device;     -   wherein the transfer die includes one or more alignment marks,         for use in a transfer-print process.

Such a transfer die can result in a photonic device with increased alignment between the photonic device and one or more parts of the precursor photonic device, which can decrease undesirable losses as light passes through them.

The transfer die may have any one or, to the extent that they are compatible, any combination of the following optional features.

The transfer die may include one or more metal patches located at least adjacent to the respective alignment marks. The one or more alignment included in the transfer die may include one or more trenches surrounding the alignment marks, which may be patterned and etched at the same time as the waveguides. Both the one or more alignment marks and one or more trenches are covered by the metal patch.

The transfer die may include a second waveguide and may include an interface which is angled relative to the guiding direction of the second waveguide. The second waveguide may include a T-bar facet.

At least one of the one or more alignment marks included in the transfer die, or both, may be formed in one of the following shapes: a rectangle, a square, an L-shape, a circle, a ring, a partial circle, and a partial ring.

The photonic device may be a III-V semiconductor device and/or the transfer die may include a sacrificial layer.

The photonic device may be a laser, a semiconductor optical amplifier, or an electro-absorption modulator. The photonic device may be an elector-absorption modulator, and the electro-absorption modulator may comprise an input waveguide and an output waveguide, and both of the input waveguide and output waveguide may comprise a port located on a same side of the transfer die.

The photonic device may be formed at least partially from indium phosphide, and/or a sacrificial layer is formed of indium gallium arsenide.

The alignment marks may be provided on an optically transparent region of the transfer die.

The transfer die may be formed on an indium phosphide substrate.

The photonic device may include one or more coarse alignment marks and one or more fine alignment marks. The one or more coarse alignment mark(s) may project in at least two non-parallel directions. The one or more coarse alignment mark(s) may be shaped as any one or more of: an arrow, a cross, a “T” shape, and an “L” shape. There may be two or more fine alignment marks which respectively project in at least two non-parallel directions. The one or more fine alignment marks may include Vernier patterns.

In a seventh aspect, embodiments of the invention provide a platform wafer, suitable for use in a transfer printing process, said platform wafer including:

-   -   one or more alignment chips, said alignment chips including one         or more alignment marks; and     -   one or more precursor photonic device(s).

Advantageously, such a platform wafer can allow larger throughput in producing photonic devices. The alignment chips can be given over solely to the provision of alignment marks.

The precursor photonic device of the seventh aspect may have any, or any combination insofar as they are compatible, of the optional features of the precursor photonic device of the fifth aspect.

In a eighth aspect, embodiments of the invention provide a transfer wafer, suitable for use in a transfer printing process, said wafer including:

-   -   one or more alignment chips, said alignment chips including one         or more alignment marks; and     -   one or more device chips.

Advantageously, such a transfer wafer can allow larger throughput in producing photonic devices. The alignment chips can be given over solely to the provision of alignment marks.

The transfer wafer may have any, or any combination insofar as they are compatible, of the optional features of the transfer die of the sixth aspect.

In a ninth aspect, embodiments of the invention provide a method of preparing a precursor photonic device comprising the steps of:

-   -   providing a wafer, comprising a substrate and a device layer;         and     -   etching one or more alignment marks into the wafer.

The method may have any one or, to the extent that they are compatible, any combination of the following optional features.

The method may further comprise: etching a cavity into the wafer, said cavity extending from an uppermost surface of the device layer to at least an uppermost surface of the substrate; and etching the one or more alignment marks into the substrate.

The method may further comprise a step of etching at least one of an input waveguide and an output waveguide, said input waveguide and/or output waveguide having a surface adjacent to the cavity. The step of etching one or more alignment marks may be performed at the same time as etching the input waveguide and/or output waveguide. Advantageously, be performing etching the alignment mark(s) and waveguide(s) at the same time, it can be ensured that there is no alignment error between the waveguide(s) and alignment mark(s).

The step of etching the one or more alignment marks and the input waveguide and/or output waveguide may comprise the steps of:

-   -   (a) providing a photoresist over an upper surface of the         precursor photonic device;     -   (b) patterning the photoresist to provide one or more exposed         regions; and     -   (c) etching the exposed regions.

The method may further comprise a step of depositing an antireflective coating, preferably formed of silicon nitride, along either or both of: one or more sidewalls; and/or a bed of the cavity. The method may further comprise a step of removing at least the antireflective coating present adjacent to the alignment marks.

The method may further comprise a step of depositing a top cladding layer over the exposed upper surface of the precursor photonic device, after the step of etching the one or more alignment marks. The method may include a step of removing portions of the top cladding layer which are within the cavity.

In a tenth aspect, embodiments of the invention provide a method of forming a transfer die, comprising the steps of:

-   -   providing a multi-layered structure, said multi-layered         structure including at least a sacrificial layer and one or more         optically active layers; and     -   etching one or more alignment marks into a part of the         multi-layered structure.

The method may have any one or, to the extent that they are compatible, any combination of the following optional features.

The step of etching one or more alignment marks may be performed concurrently with the step of etching one or more device structure into the multi-layered structure. Advantageously, by etching the alignment mark(s) and device structure concurrently, it can be ensured that there is no alignment error between the device structure and the alignment mark(s). The method may include a step of depositing a stress compensation layer.

Etching the one or more alignment marks into a part of the multi-layered structure may include etching a region of the transfer die such that it is optically transparent.

The alignment marks may be etched entirely through the multi-layered structure.

Further aspects of the present invention provide: a computer program comprising code which, when run on a computer, causes the computer to perform the method of the first, ninth, or tenth aspects; a computer readable medium storing a computer program comprising code which, when run on a computer, causes the computer to perform the method of the first, ninth, or tenth aspects; and a computer system programmed to perform the method of the first, ninth, or tenth aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows an example alignment mark set used in embodiments of the present invention;

FIGS. 2A and 2B show respective views of a precursor photonic device, comprising a silicon-on-insulator, SOI, waveguide including alignment marks;

FIGS. 3(i)-3(ix)(B) show various manufacturing steps of a method to produce an SOI waveguide including alignment marks;

FIGS. 4(A)-4(D) show respective views of an SOI waveguide manufactured using the method of FIGS. 3(i)-3(ix);

FIGS. 5(i)-5(x) show various manufacturing steps of a variant method to produce an SOI waveguide including alignment marks;

FIG. 6A shows an alignment mark for a transfer die including a III-V based device;

FIG. 6B shows the alignment mark of FIG. 6A, when positioned over a corresponding alignment mark in a precursor photonic device;

FIGS. 7(i)-7(xii) show various manufacturing steps of a method to produce a transfer die including a III-V based laser;

FIG. 8 shows a top-down view of a transfer die including a III-V based laser fabricated according to the method of FIGS. 7(i)-7(xii);

FIGS. 9(i)-9(xiii) show various manufacturing steps of a variant method to produce a transfer die including a III-V based laser;

FIG. 10 shows a top-down view of a transfer die including a laser produced using the method of FIGS. 9(i)-9(xiii);

FIGS. 11(i)-11(xiv) show various manufacturing steps of a method to produce a transfer die including a III-V based electro-absorption modulator, EAM;

FIG. 12 shows a top-down view of a transfer die including an EAM produced using the method of FIGS. 11(i)-11(xiv);

FIGS. 13(i)-13 (xiv) show various manufacturing steps of a variant method to produce a transfer die including a III-V based EAM;

FIG. 14 shows a top-down view of a transfer die including an EAM produced using the method of FIGS. 13(i)-13(xiv);

FIGS. 15(A) and 15(B) show respective views of a III-V/Si based laser after micro-transfer printing;

FIGS. 16(A) and 16(B) show respective views of a III-V/Si based EAM after micro-transfer printing;

FIGS. 17(A) and 17(B) show respective views of a variant III-V/Si based laser after micro-transfer printing;

FIGS. 18(A) and 18(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing;

FIGS. 19(A) and 19(B) show respective views of a variant III-V/Si based laser after micro-transfer printing;

FIGS. 20(A) and 20(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing;

FIGS. 21(A) and 21(B) show respective views of a variant III-V/Si based laser after micro-transfer printing;

FIGS. 22(A) and 22(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing;

FIG. 23 shows a variation in the placement of alignment marks;

FIG. 24 shows a further variation in the placement of alignment marks;

FIG. 25 shows a further variation in the placement of alignment marks;

FIG. 26 shows a further variation in the placement of alignment marks;

FIG. 27 shows an alignment token die;

FIG. 28 shows a transfer wafer including III-V devices;

FIG. 29 shows a platform wafer including precursor photonic devices;

FIG. 30 shows a further variation of an optoelectronic device;

FIG. 31 shows in more detail the alignment mark(s) of the transfer die;

FIG. 32 shows in more detail the coupling interface of FIG. 30;

FIG. 33 shows a further variation of an optoelectronic device;

FIG. 34 shows a further variation of an alignment mark used on a transfer die;

FIG. 35 shows a further variation of an alignment mark used on a precursor photonic device;

FIG. 36 shows a further variant of an alignment mark used on a precursor photonic device;

FIG. 37 shows in more detail the coupling interface of FIG. 33;

FIG. 38 shows a further variation of an optoelectronic device; and

FIGS. 39A-39D show variations of alignment marks.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art.

FIG. 1 shows an example alignment mark set used in embodiments of the present invention. The alignment mark set includes four components: fine alignment marks present on the transfer die 101; fine alignment marks present on the precursor device 102; a coarse alignment mark on the transfer die 103; and a coarse alignment mark on the precursor device 104.

The coarse alignment marks take the form of a + or cross shape, with the alignment mark on the precursor device 104 being larger than that on the transfer die 103. Therefore, when the transfer die is positioned above the precursor device, an outline of the coarse alignment mark of the precursor device can be seen surrounding the coarse alignment mark of the transfer die. This alignment is typically accurate to within 2 μm. The coarse alignment marks make take essentially any shape, for example a ‘T’ shape, ‘L’ shape, or any other shape which extends in at least two non-parallel directions. Preferably, the coarse alignment marks take the form of a cross or + shape.

The fine alignment marks take the form of a set of Vernier scales. The Vernier scales on each of the precursor device and transfer die allow for alignment accuracy to within the tens to hundreds of nanometres, as shown.

FIGS. 2A and 2B show respective views of a precursor photonic device 200. The device comprises a silicon-on-insulator, SOI, waveguide including alignment marks. FIG. 2A is a top-down view of the precursor photonic device 200, where ‘x’ is the guiding direction of light through the device, ‘y’ is a height direction out of the plane, and ‘z’ is a direction perpendicular to ‘x’ and ‘y’. The device includes an input waveguide 201, formed from a silicon-on-insulator wafer. Opposite the input waveguide, across a cavity 203, is output waveguide 204. It should be noted that the prefix input and output in this context is for clarity, and that, as discussed below, either waveguide 201 or 204 may function as the input or output waveguide and vice versa. The waveguides may be either 1 μm or 3 μm in height (as measured from a bottom cladding layer to a top cladding layer of the waveguide), and the mode centre may be aligned with the waveguide mode of a photonic device to be placed in the cavity, as is discussed in more detail below. The cavity 203 extends across a width of the device, between the input 201 and output 204 waveguides, and contains four alignment marks 205 of the type discussed previously in relation to the precursor device. The upper surface 202 of the device can be seen, which is higher (i.e. closer to the viewer of the figure) than the bed of cavity 203.

FIG. 2B shows a section view of the precursor photonic device 200 along the cut A-A′. It should be noted that the view is not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 2A in order to fit to the page. More of the structure of the device can be seen here, for example upper cladding layer 208 and lower cladding layer 209 (provided by the buried oxide/insulator layer of the SOI wafer) can be seen. As well as passivation layer 207.

Notably, the cavity 203 sidewalls of the cavity adjacent to the input 201 and output 204 waveguides are lined with an anti-reflective coating 206. In this example, the antireflective coating is formed of silicon nitride, and specifically Si₃N₄. As can be seen, the alignment features 205 are provided within the bed of the cavity and extend further into substrate 210 than the remaining bed of the cavity. The extension of the alignment marks into the bed of the cavity should be sufficient as to provide enough contrast for alignment by microscope (e.g. optical microscope). For example, the alignment marks may extend at least 150 nm and no more than 2000 nm into the bed of the cavity. The bed of the cavity where the alignment marks are not present may have a surface roughness of less than or equal to 1 nm (for example, the arithmetic average value Pa as measured according to ISO 25178). This increases the reliability of subsequent bonding.

FIGS. 3(i)-3(ix) show various manufacturing steps of a method to produce an SOI waveguide, an example of the precursor photonic device 200, including alignment marks. In a first step shown in FIG. 3(i), an SOI wafer is provided which includes: device or silicon on insulator layer 301, buried oxide layer 302, and substrate 303. Next, in a step shown in FIG. 3(ii), a cavity patterning layer 304 is deposited over the upper surface of the device layer 301. This cavity patterning layer is formed of SiO₂.

After the cavity patterning layer 304 is provided, a cavity 203 is patterned and etched as shown in FIG. 3(iii). The depth of the cavity may depend on the mode centre alignment between the waveguides 201/204 and the optical modes in the device to be bonded to the bed of the cavity. Alternatively, the height of various layers in the device to be bonded may be adjusted to align the mode centre between the waveguide(s) and the device. After the cavity 203 is etched, an antireflective coating (ARC) layer 206 is provided over all exposed surfaces. Therefore both the sidewalls and bed of the cavity 203 are lined with ARC layer, which in some examples is formed of silicon nitride e.g. Si₃N₄. In most examples, the bed and upper surface of the device will be cleared of their ARC layers.

Next, in a step shown in FIG. 3(v)(A) which is a top-down view, a photoresist 305 is provided to define both waveguides 201/204 as well as alignment marks 307. As can be seen in FIG. 3(v)(B), a cross-section along the line A-A′, the photoresist resides over only a portion of the ARC layer provided previously, leaving exposed portions either side of the photoresist. The width of the photoresist defines the width of the resulting waveguide.

FIG. 3(v)(C) shows a cross-section taken along the line B-B′, and indicates that the photoresist extends down the sidewall of the cavity and defines the alignment marks 307 to be etched. This can be a two-step process, where the photoresist is placed and then patterned through photolithography or similar.

After the photoresist 305 is provided and patterned, the exposed portions of the device are etched to define: the waveguides 201 and 204, as well as the alignment marks 205. The result of this etching step is shown in FIG. 3(iv)(A), a top down view 3(iv)(B) a cross-section taken along A-A′, and 3(iv)(C) a cross-section taken along B-B′. This etch also defines the uppermost surface 202 of the remaining device layer. Typically, the uppermost surface 202 is a portion of the device layer and so formed from silicon. However this can be covered with silicon dioxide or silicon nitride. In the cavity 203, the etch extends through the antireflective coating layer located over the bed of the cavity, and partially into the silicon substrate. As has been discussed previously, the depth into the cavity should be sufficient to provide contrast when viewed under an optical microscope, for example between 150 nm and 2000 nm.

Next, in a step shown in FIGS. 3(vii)(A) and 3(vii)(B), corresponding to cross-sections taken along the lines A-A′ and B-B′ discussed previously, passivation layer 207 is provided over all exposed surfaces of the device. In this example, the passivation layer is formed of silicon dioxide, SiO₂.

A further photoresist 305 is provided, as shown in FIGS. 3(viii)(A) and 3(viii)(B) which are again corresponding to cross-sections taken along the lines A-A′ and B-B′ respectively. The photoresists masks all portions of the device bar the sidewalls of the cavity 203 and the bottom of the cavity. The figures show the device after an etched is performed, using the further photoresist 350 as a mask. The etch removes the passivation layer 207 provided within the cavity.

After the passivation layer 207 within the cavity is removed, yet a further photoresist 305 is provided and the antireflective coating present on the bed of the cavity 203 is removed. This photoresist, and the result of the etch, is shown in FIGS. 3(ix)(A) and 3(ix)(B), again cross-sections taken along the lines A-A′ and B-B′. The photoresist is then removed.

This is the final step in the fabrication process, and results in a precursor photonic device 200 as shown in FIG. 4(A). Where this figure shares features with those previously described, like features are indicated by like reference numerals. FIGS. 4(A) and 4(D) mirror FIGS. 2A and 2B respectively. FIGS. 4(B) and 4(C) are cross-sections of FIG. 4(A) taken along the lines A-A′ and B-B′ respectively.

FIGS. 5(i)-5(x) show various manufacturing steps of a variant method to produce an SOI waveguide including alignment marks. As before, in FIG. 5(i) an SOI wafer is provided formed of a device layer 301, buried oxide layer 302, and silicon substrate 303. The device layer is either 1.0 μm tall or 3.0 μm tall (measured from an uppermost surface of the buried oxide layer to an uppermost surface of the device layer).

Next, in a step shown in FIG. 5(ii), an upper cladding layer 208 and photoresist 501 is provided over a portion of the waveguide. The photoresist extends across the length of the device, i.e. into/out of the plane of FIG. 5(ii). An etch is then performed, using the photoresist 501 as a mask. The result of the etch is shown in FIG. 5(iii) where waveguide 201 has been defined, where the upper cladding is provided by upper cladding layer 208 and the lower cladding is provided by buried oxide layer 302.

After the etch, the photoresist 501 is removed and further upper cladding material 208 is provided over all exposed surfaces of the device. The result of this is shown in FIG. 5(iv). This can be provided either through thermal oxidation of the silicon device layer, or through deposition (e.g. blanket) of silicon dioxide.

FIG. 5(v) shows the same structure as FIG. 5(iv), but in a cross-sectional view rotated by 90° as indicated by the coordinate marker. The cross-sectional view is taken through the waveguide formed in the previous etching step, and so light is guided from the left hand side of the figure to the right (or vice versa).

After the upper cladding layer has been provided, a cavity 203 of the type previously mentioned is etched into the device. The result of this is shown in FIG. 5(vi). After the cavity 203 has been etched, an antireflective coating 206 is deposited in much the same manner as discussed previously. The structure including the antireflective coating is shown in FIG. 5(vii). Alignment marks 205 are then etched in the bed of the cavity 203, in the manner discussed previously (noting however, that the waveguides 201/204 have already been formed). In FIG. 5(ix), a further photoresist 501 is provided for removing the antireflective coating present in the bed of the cavity. This is done to improve the bonding cohesion. The result of this removal is shown in FIG. 5(x), which is also a final step in the preparation of the precursor photonic device.

FIG. 6A shows an alignment mark for a III-V transfer die. Region 601, outside of the periphery of the alignment mark is the III-V chip or die on which the alignment mark resides. The region 602 within the alignment mark is transparent. This can be achieved, for example, by thinning the region until light can be transmitted through it or removing this region entirely.

FIG. 6B shows the alignment mark of FIG. 6A, when positioned over a corresponding alignment mark in a precursor photonic device. The alignment mark 205 in the precursor photonic device is used in combination with the alignment mark in the III-V transfer die to facilitate alignment in the manner shown.

FIGS. 7(i)-7(xii) show various manufacturing steps of a method to produce a transfer die including a III-V based laser. In a first step, shown in FIG. 7(i), a III-V semiconductor stack 701 is provided. The stack comprises, going from an uppermost layer (i.e. distalmost to the substrate) first to a lowermost:

702—P doped indium gallium arsenide (P—InGaAs) layer;

703—P doped indium phosphide (P—InP) layer;

704—Aluminium indium gallium arsenide (AlInGaAs) multiple quantum well layer;

705—N doped indium phosphide (N—InP) layer;

706—Indium gallium arsenide (InGaAs) sacrificial layer; and

707—Indium phosphide substrate.

The stack may include greater or fewer numbers of layers. In a particular example, the stack comprises the following layers:

Layer R n/u/p Material Thickness (nm) Eg (nm) Doping (10¹⁸) Dopant 15 1 p InGaAs 400 1499.98 1 Zn 14 1 p InGaAsP 50 1302.91 1.5 Zn 13 1 p InP 1340 918.407 1 Zn 12 1 p InGaAsP 20 1302.91 1 Zn 11 1 p AlInGaAs 60 843.435 1 C 10 1 uid AlInGaAs 70 968.035 — — 9 12x uid AlInGaAs 7 1127.14 — — 8 12x Active AlInGaAs 9 1278.2 — — 7 1 uid AlInGaAs 7 1127.14 — — 6 1 uid InGaAsP 77 1100 — — 5 1 n InP 80 918.407 0.2 Si 4 1 n InP 70 918.407 0.5 Si 3 1 n InP 920 918.407 0.8 Si 2 1 n InGaAs 1000 1499.98 1 Si 1 Substrate: semi-insulating and n doped InP

After the stack 701 is provided, a mask 709 (formed, for example, from SiO₂) a photoresist 708 is disposed and patterned so as to define: alignment area 710, n contact area 711, and alignment area 712. That is, these are the regions where these structures will be formed after subsequent processing. The areas of the stack not covered by the photoresist are then etched down to the N—InP layer, as shown in FIG. 7(iii).

The mask 709 is reapplied, so as to cover the freshly exposed surfaces, and a further photoresist 708 is then provided, so as to define: a waveguide, and one or more alignment marks 713. This is shown in FIG. 7(iv). The waveguide 714 and alignment marks 715 are then etched, as is shown in FIG. 7(v). Notably, in this example, the alignment mark etch does not extend all of the way through the N—InP layer. This helps improve the bond quality when the III-V transfer die is bonded to the precursor photonic device. As the waveguide 714 and alignment mark(s) 715 are patterned and etched at the same time, there is no alignment error between the waveguide and alignment marks on the III-V transfer die.

After the etching step shown in FIG. 7(v), upper cladding layer 716 is provided over the exposed upper surfaces of the device. The portion(s) of the upper cladding layer overlapping with the alignment marks are then removed e.g. through a wet etch. This is shown in FIG. 7(vi).

Next, further silicon dioxide (providing further upper cladding layer) is deposited as well as any stress compensation layers required (not shown, for clarity). Notably, in this example, the cladding layer 716 does not extend further towards the substrate than the N—InP. This can improve the bonding cohesion. After this, vias are opened for the p and n contacts. The results of these steps are shown in FIG. 7(vii). In this example, a via 718 is opened for the p contact the P—InGaAs layer on an upper surface of the waveguide 714. A separate via 717 is opened for the n contact which extends into the N—InP layer. Of course, if the layers were swapped with respect to their dopants, the n and p contacts would be swapped as well.

After the vias are opened, a metallization process is performed to provide electrodes 719 and 720 which contact the p and n doped regions of the stack respectively. The result of this is shown in FIG. 7(viii).

Next, a hard mask is deposited over the upper surface, and patterned to allow the etching of facets of the device. This provides relatively clean interfaces or facets at the extremities of the stack, which reduces optical losses when bonded to the precursor photonic device. The results of these steps are shown in FIG. 7(ix).

To prepare the device for micro-transfer printing, the hard mask (formed in this example of SiO₂) which remains outside of the chip (left and right hand edges) are removed. This is shown in FIG. 7(x). This exposes the upper surface of the InGaAs sacrificial layer 706 around a perimeter of the stack. A lifting or micro-transfer print photoresist 721 is then applied to the upper surface of the stack, to prepare the stack to be lifted off of the substrate. The stamp, used in the micro-transfer printing process, may be formed of Polydimethylsiloxane (PDMS) A final step in the preparation of the III-V transfer die is to etch away the sacrificial layer, leaving the transfer die connected to the InP substrate by tethers 722. The die is then ready for micro-transfer printing.

FIG. 8 shows a top-down view of a transfer die including a III-V based laser fabricated according to the method of FIGS. 7(i)-7(xii). Notably, the alignment features 715 formed previously are visible when looking down through the device. The area surrounding the alignment features is transparent to allow optical alignment with corresponding features on the precursor photonic device. FIG. 8 also shows the A-A′ line which all of FIGS. 7(i)-7(xiii) are cross-sections along.

FIGS. 9(i)-9(xiii) show various manufacturing steps of a variant method to produce a transfer die including a III-V based laser.

As before, a III-V semiconductor based stack 701 is provided in a step shown in FIG. 9(i). A hard mask 708 is provided over the uppermost surface. Subsequently, photoresist 708 is then provided, to define the chip or die width (left and right hand side portions of photoresist 708) as well as to define the waveguide. This is shown in FIG. 9(ii).

After the photoresist and hard mask are provided, the portions of the stack not covered by the photoresist are etched down to at least the P—InP layer. In the example shown in FIG. 9(iii), the etch is performed so that only a relatively thin layer of P—InP remains. After this etch, upper cladding layer 716 (formed of silicon dioxide) is deposited, patterned, and etched so as to provide n contact area 711 and alignment areas 710 and 712. Chip boundary areas 902 are defined around the stack. The result of this is shown in FIG. 9(iv).

Next, further upper cladding layer is deposited, patterned, and etched so as to expose the sacrificial layer in the alignment area(s) 710 and chip boundary 902. A wet etch is preferred here. The results of these steps are shown in FIG. 9(v). The etch may extend down to an upper surface of the sacrificial layer. In some examples, a thin portion of the N—InP layer may be retained. After the etch is performed, further upper cladding 716 is provided over all exposed surfaces. The upper cladding layer has a thickness of around 500 nm. Stress compensation layers may also be provided at this stage if required. The device with upper cladding layer provided is shown in FIG. 9(vi).

As before, after the upper cladding layer is provided vias 717 and 718 are opened in it for the electrodes. A first via 717 is provided which exposes an upper surface of the N—InP layer, and a second via 718 is provided which exposes an upper surface of the P—InGaAs layer. This is shown in FIG. 9(vii). After the vias are provided, a metallization process is performed which provides electrodes 719 and 720 as discussed previously.

After the electrodes are formed, alignment mark(s) 904 are deposited within the alignment regions formed previously. In this example, instead of alignment marks being formed by etching the N—InP layer, they are formed by deposition of a metal such as titanium nitride which is then dry etched to provide the features of the alignment marks (e.g. coarse and fine alignment marks). The result of this is shown in FIG. 9(ix).

Next, a hard mark is deposited, patterned, and facets are etched. The facets are then coated with silicon dioxide, as are the alignment marks 904 formed previously. The results of these steps are shown in FIG. 9(x). After this, the silicon dioxide located outside of the chip boundaries (i.e. around a periphery of the III-V transfer die) is removed to expose an upper surface of the InGaAs sacrificial layer 706. As before, a lifting or print photoresist 906 is then provided over the exposed surfaces of the III-V transfer die. FIG. 9(xii) shows the transfer die with the photoresist applied.

In a final step, the sacrificial InGaAs layer is etched away, leaving the III-V transfer die suspended by tethers 722 and ready for micro-transfer printing.

FIG. 10 shows a top-down view of a transfer die including a laser produced using the method of FIGS. 9(i)-9(xiii). Of note are alignment mark(s) 904 in the four corners of the III-V transfer die. FIG. 10 also shows the A-A′ line which all of FIGS. 9(i)-9(xiii) are cross-sections along.

FIGS. 11(i)-11(xiv) show various manufacturing steps of a method to produce a transfer die including a III-V based electro-absorption modulator, EAM. In a first step, shown in FIG. 11(i) a III-V semiconductor based stack 701 is provided, as before. Next, a photoresist 1101 is provided and the exposed regions etched to provide alignment area 1102 and 1103, as shown in FIG. 11(ii)

FIG. 11(iii) shows the results of the next steps, where a hard mask is provided, patterned and etched to define the waveguide 714 as well as chip boundary areas 1104 and to provide trenches within the alignment areas 1102 and 1103. Again, as the waveguide and alignment features are patterned and etched at the same time, there are no alignment errors therebetween.

Next, upper cladding layer 716, which may be formed of silica or silicon dioxide, is provided and then patterned and etched in the alignment areas 1102 1103 down through the N—InP layer as shown in FIG. 11(iv). This is performed via a wet etch. The etch may extend through to an upper surface of the InGaAs sacrificial layer, but preferably retains a few nanometres of the N—InP layer to enhance bonding adhesion.

FIG. 11(v) shows the device after further silicon dioxide has been provided, and the p-contact 719 is formed to contact the top of the waveguide 714. Again, stress compensations layers can be provided at this time if required. After the p-contact 719 is provided, n-contact 720 is also provided electrically connecting to the N—InP layer adjacent to the waveguide 714, as shown in FIG. 11(vi).

Isolation area 1105 is then etched in a portion of the device adjacent to the waveguide 714. This is performed through a dry etch, and then a wet etch of the InP layer using the InGaAs sacrificial layer as a wet etch stop. This is shown in FIG. 11(vii).

Silicon nitride 1106 is then deposited over all exposed surfaces, and a Benzocyclobutene (BCB) fill 1107 deposited as shown in FIG. 11(viii). The BCB fill is then etched so that upper portions of the silicon nitride 1106 layer are exposed. Next, as shown in FIG. 11(ix), the upper surface (asides from the p electrode 719) is planarized. A via 1108 is opened to allow electrical connection to the n electrode 720.

Next, in a step shown in FIG. 11(x), the p-electrode trace 1109 is provided over the upper surface of the device. At the same time, alignment marks 1115 are etched. After this, the n-electrode trace 1110 is provided, electrically contacting the n electrode 720 and extending through via 1108.

A hard mask 1111 is then deposited, patterned, and facets etched. The traces 1109 and 1110 are left exposed. The chip boundary is also etched, as shown in FIG. 11(xii). After this, lifting or print photoresist 1112 is provided over the exposed surfaces of the III-V transfer die, as shown in FIG. 11(xiii). Finally, the sacrificial layer is etched away, such that the transfer die is suspended by tethers 722 as shown in FIG. 11(xiv). The transfer die is then ready for micro-transfer printing.

FIG. 12 shows a top-down view of a transfer die including an EAM produced using the method of FIGS. 11(i)-11(xiv). All of FIGS. 11(i)-11(xiv) are sections along the cut A-A′. As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 12 in order to fit to the pages.

FIGS. 13(i)-13 (xiv) show various manufacturing steps of a variant method to produce a transfer die including a III-V based EAM. In a first step, as shown in FIG. 13(i), a III-V semiconductor stack 701 is provided. A photoresist 1301 is then applied, to define waveguide 714, as well as alignment areas 1102 and 1103 and p and n contact areas 1113 and 711. The edges of photoresist 1301 defines the chip boundary.

The device is then etched, as shown in FIG. 13(iii), thereby providing waveguide 714 and alignment areas 1102 and 1103. Next, an upper cladding layer (e.g. formed of silicon dioxide) is provided as well as any stress compensation layers that may be required. The P-contact 719 is also disposed in contact with the P—InGaAs layer of the stack. This is shown in FIG. 13(iv). Next, N-contact 720 is provided which is electrically connected to the N—InP layer of the stack, as illustrated in FIG. 13(v).

After the electrical contacts have been provided, an isolation area 1105 is etched. At the same time, the alignment areas are etched down to the InGaAs sacrificial layer as illustrated in FIG. 13(vi). As before, this etch is performed through a combination of a dry etch and wet etch to remove the InP, using the InGaAs layer as a wet etch stop.

As with the previous method, a silicon nitride layer 1106 is then deposited followed by a BCB fill. This is shown in FIG. 13(vii). Again, the device (with the exception of the p-contact 719) is planarized and a via 1108 for electrode connection is opened through the BCB fill. The result of these steps is shown in FIG. 13(viii).

Electrical trace 1109 for the p-contact 719 is then made, as shown in FIG. 13(ix), and electrical trace 1110 for the n-contact is made as shown in FIG. 13(x). The alignment marks 1302 are then made, in this instance through the deposition of a relatively thin metal layer (e.g. TiN) which is patterned and etched to produce the fine and coarse features discussed above. This is shown in FIG. 13(xi). In an optional step, trenches 1303 may be etched in the BCB fill first for the alignment markers to be disposed in. Alternatively, the alignment markers may be etched into the BCB fill itself. FIG. 13(xi′) shows the optional variant in which trenches 1303 are etched.

After the alignment marks have been made, a hard mask 1111 is disposed over the III-V transfer die (although electrode traces 1109 and 1110 are left exposed). Subsequently, the hard mask is patterned and facets are etched, the facets are coated and the chip boundaries are etched. This is shown in FIG. 13(xii).

As before, a transfer photoresist 1112 is then provided over the upper exposed surfaces of the III-V transfer die, as shown in FIG. 13(xiii). The sacrificial InGaAs layer can then be etched away, leaving the III-V transfer die suspended from tethers 722 as shown in FIG. 13(xiv).

FIG. 14 shows a top-down view of a transfer die including an EAM produced using the method of FIGS. 13(i)-13(xiv). All of FIGS. 13(i)-13(xiv) are sections along the cut A-A′. As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 14 in order to fit to the pages.

After the precursor photonic device (e.g. shown in FIG. 2A) has been produced, and the transfer die containing a photonic device (preferably including a III-V semiconductor based optically active region) has been completed, the two can be integrated. In one example, the integration process comprises the following steps:

For the precursor photonic device, e.g. the SOI waveguide platform:

-   -   Plasma treatment of the SOI waveguide wafer for approximately 30         seconds;     -   Dipping of the SOI waveguide into purified or deionised water;         and     -   Spin drying of the SOI waveguide wafer (or spin rinse drying         without drying gas).

For the III-V transfer die/chip/wafer:

-   -   Plasma treatment of the III-V die for approximately 30 seconds.

After the device and transfer die have been pre-treated, a micro-transfer print technique is used to align the III-V transfer die in the cavity of the precursor photonic device. This alignment utilises the alignment marks on each of the transfer die and precursor photonic device to enhance the alignment accuracy.

After the devices have been aligned, and the III-V transfer die ‘printed’ onto the precursor photonic device, an annealing process is used to facilitate bonding therebetween. Preferably, the annealing process includes annealing the assembled III-V/Si wafer at 300° C. for around 30 minutes in N₂ gas. This has been found to reliability bond the III-V layer(s) in the photonic device to the cavity of the precursor photonic device.

FIGS. 15(A) and 15(B) show respective views of a III-V/Si based laser after micro-transfer printing and bonding process. The III-V based laser is that manufactured according to FIGS. 7(i)-7(xii) and shown in FIG. 8. As can be seen, the alignment features 715 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is a laser, the waveguide 204 is not intended for use and so a sizable gap can be left between the III-V device and the output waveguide. Indeed, in this example, the precursor photonic device may have only one waveguide, waveguide 201 which functions as an output waveguide for the laser. FIG. 15(B) is a section view taken along the A-A′ line of FIG. 15(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 15(A) in order to fit to the page.

FIGS. 16(A) and 16(B) show respective views of a III-V/Si based EAM after micro-transfer printing. The III-V based EAM is that manufactured according to FIGS. 11(i)-11(xiv) and shown in FIG. 12. As can be seen, the alignment features 1115 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is an EAM, both waveguides 201 and 204 are intended for use and so the gap between the III-V device and the waveguides should be kept relatively small.

FIG. 16(B) is a section view taken along the A-A′ line of FIG. 16(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 16(A) in order to fit to the page.

FIGS. 17(A) and 17(B) show respective views of a variant III-V/Si based laser after micro-transfer printing. As can be seen, the alignment features 715 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is a laser, the waveguide 204 is not intended for use and so a sizable gap can be left between the III-V device and the output waveguide. Indeed, in this example, the precursor photonic device may have only one waveguide, waveguide 201 which functions as an output waveguide for the laser. The device in FIGS. 17(A) and 17(B) differs from that in 15(A) and 15(B) in that the gap between waveguide 201 and the III-V device is filled with a bridge waveguide 1701 (formed, in this example, from either Si₃N₄ or amorphous silicon, α-Si). This bridge waveguide can provide an index match between the waveguide 201 and III-V device, and so further reduce any optical losses as light moves from the III-V based laser to the waveguide 201.

FIG. 17(B) is a section view taken along the A-A′ line of FIG. 17(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 17(A) in order to fit to the page.

FIGS. 18(A) and 18(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing. As can be seen, the alignment features 1115 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. The device in FIGS. 18(A) and 18(B) differs from that in 16(A) and 16(B) in that the gaps between waveguides 201 and 204 and the III-V device are both filled with bridge waveguides 1701 (formed, in this example, from either Si₃N₄ or amorphous silicon, α-Si). These bridge waveguides can provide an index match between the waveguides 201 and 204 and the III-V device, and so further reduce any optical losses as light moves from the input waveguide 201 into the III-V based EAM and on to the output waveguide 204.

FIG. 18(B) is a section view taken along the A-A′ line of FIG. 18(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 18(A) in order to fit to the page.

FIGS. 19(A) and 19(B) show respective views of a variant III-V/Si based laser after micro-transfer printing. The III-V based laser is that manufactured according to FIGS. 9(i)-9(xiii) and shown in FIG. 10. As can be seen, the alignment features 904 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is a laser, the waveguide 204 is not intended for use and so a sizable gap can be left between the III-V device and the output waveguide. Indeed, in this example, the precursor photonic device may have only one waveguide, waveguide 201 which functions as an output waveguide for the laser.

FIG. 19(B) is a section view taken along the A-A′ line of FIG. 19(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 19(A) in order to fit to the page.

FIGS. 20(A) and 20(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing. The III-V based EAM is that manufactured according to FIGS. 13(i)-13(xiv) and shown in FIG. 14. As can be seen, the alignment features 1302 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is an EAM, both waveguides 201 and 204 are intended for use and so the gap between the III-V device and the waveguides should be kept relatively small.

FIG. 20(B) is a section view taken along the A-A′ line of FIG. 20(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 20(A) in order to fit to the page.

FIGS. 21(A) and 21(B) show respective views of a variant III-V/Si based laser after micro-transfer printing. As can be seen, the alignment features 904 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is a laser, the waveguide 204 is not intended for use and so a sizable gap can be left between the III-V device and the output waveguide. Indeed, in this example, the precursor photonic device may have only one waveguide, waveguide 201 which functions as an output waveguide for the laser. The device in FIGS. 21(A) and 21(B) differs from that in 19(A) and 19(B) in that the gap between waveguide 201 and the III-V device is filled with a bridge waveguide 1701 (formed, in this example, from either Si₃N₄ or amorphous silicon, α-Si). This bridge waveguide can provide an index match between the waveguide 201 and III-V device, and so further reduce any optical losses as light moves from the III-V based laser to the waveguide 201.

FIG. 21(B) is a section view taken along the A-A′ line of FIG. 21(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 21(A) in order to fit to the page.

FIGS. 22(A) and 22(B) show respective views of a variant III-V/Si based EAM after micro-transfer printing. The III-V based EAM is that manufactured according to FIGS. 13(i)-13(xiv) and shown in FIG. 14. As can be seen, the alignment features 1302 of the transfer die and the alignment features 205 of the precursor device overlap such that they can be used in the accurate alignment of one component relative to the other. In this example, as the III-V device is an EAM, both waveguides 201 and 204 are intended for use and so the gap between the III-V device and the waveguides should be kept relatively small. The device in FIGS. 22(A) and 22(B) differs from that in 20(A) and 20(B) in that the gap between waveguides 201 and 204 and the III-V device is filled with a bridge waveguide (formed, in this example, from either Si₃N₄ or amorphous silicon, α-Si). These bridge waveguides can provide an index match between the waveguides 201 and 204 and the III-V device, and so further reduce any optical losses as light moves from the input waveguide 201 into the III-V based EAM and on to the output waveguide 204.

FIG. 22(B) is a section view taken along the A-A′ line of FIG. 22(A). As before, it should be noted that the views are not to scale, and certain regions have been compressed relative to the dimensions shown in FIG. 22(A) in order to fit to the page.

FIG. 23 shows a variation in the placement of alignment marks. In the example shown, the alignment marks (formed of fine 2301 and coarse 2302 alignment marks) in the precursor photonic device are made within the cavity 103 but outside of the perimeter of the III-V transfer die i.e. outside of the area of the cavity which will form the bonding area. Advantageously, this means that there are no alignment marks etched in the bonding area, which could improve the bond adhesion between the III-V transfer die and the cavity. Further, the alignment marks on the III-V transfer die would not need to be transparent, as they merely abut the alignment marks of the precursor photonic device. As a further advantage, the alignment marks in the precursor device may have a very similar or equal height, which makes imaging simpler. The fine alignment marks 2301 can be placed on two, three, or four sides of the III-V transfer die and corresponding locates in the SI cavity.

FIG. 24 shows a further variation in the placement of alignment mark. In the example shown, the alignment marks (formed of fine 2301 and coarse 2302 alignment marks) in the precursor photonic device are made within the top silicon layer 202 of the device adjacent to the cavity instead of in the cavity itself. This allows the alignment marks to be easily self-aligned with the waveguide, because they can be etched in the same step as the waveguide. Again, the alignment marks in precursor photonic device can have a very similar or equal height to those in the III-V transfer die which facilitates imaging. The arrows indicated in the image demonstrate the edge alignment clearance, which is a factor in this example.

FIG. 25 shows a further variation in the placement of alignment marks. The example shown is a combination of the examples in FIGS. 23 and 24, in that alignment marks are present both in the top silicon layer 202 of the precursor photonic device and in the cavity 103. This combinations the advantages discussed in relation to FIGS. 23 and 24, allowing for two edges of the cavity to be far away from the III-V transfer die whilst allowing for alignment using either or both of the marks on the edge of the cavity or on the bottom of the cavity.

FIG. 26 shows a further variation in the placement of alignment marks. Here the III-V transfer die is not rectangular, and so alignment marks can be placed on additional sides within the cavity 103. Whilst not shown, a U-bend version of the EAM could also be used, in which case the waveguide enters and exits the chip on the same side of the III-V transfer die and only one edge of the III-V transfer die would need to be in close proximity (i.e. an effective optical coupling distance) from the cavity edge in the precursor photonic device.

FIG. 27 shows an alignment token die or chip 2700. The alignment token die does not include a III-V based device, and instead only includes alignment features and some form of substrate in or on which they are located. In the context of wafer based operations (discussed below), these alignment die can be used to substitute some of the III-V transfer die containing photonic devices so that none of the III-V transfer die include alignment features. This allows the token die's shape and size, as well as the design and location of the alignment marks, to be purely optimised to maximise alignment without having to also provide a working photonic device on the die. For example, the fine Vernier patterns can be made larger, allowing for improved alignment accuracy and precision.

FIG. 28 shows a III-V transfer wafer 2800 including a plurality of the alignment token die 2700 as shown in FIG. 27. In this example, the alignment token die 2700 are located in four corners of the wafer and in a central portion. The remaining spaces are populated by III-V transfer die 2802 including photonic devices. These III-V transfer die may not have alignment features themselves, as alignment of the III-V transfer wafer with the platform wafer can be achieved through use of the alignment token die 2700. However, the III-V transfer die 2802 may also have alignment marks. For example, the alignment token die 2700 may have only coarse alignment marks. In one example, the alignment marks in the alignment token die 2700 may be sufficient for ‘by eye’ (i.e. unmagnified) alignment and the alignment marks present in the III-V transfer die 2802 may be used for fine alignment (requiring a degree of magnification).

FIG. 29 shows a platform wafer 2900 including a plurality of alignment token die 2700 as shown in FIG. 27. In this example, the alignment token die 2700 are located in four corners of the wafer and in a central portion. The remaining spaces are populated by precursor photonic devices 2902 such as those shown in FIG. 2A. These precursor photonic devices may not have alignment features themselves, as alignment of the platform wafer with the III-V transfer wafer can be achieved through use of the alignment token die 2700. However, the precursor photonic devices 2902 may also have alignment marks. For example, the alignment token die 2700 may have only coarse alignment marks. In one example, the alignment marks in the alignment token die 2700 may be sufficient for ‘by eye’ (i.e. unmagnified) alignment and the alignment marks present in the precursor photonic devices 2902 may be used for fine alignment (requiring a degree of magnification).

In use, a stamp such as that discussed previously, may be used to lift the III-V transfer die 2802 as well as alignment token die 2700 from the III-V transfer wafer. The stamp is then aligned with alignment token die 2700 on the platform wafer 2900, such that the photonic devices in the III-V transfer die can be printed onto the platform wafer 2900.

FIG. 30 shows a further variation of an optoelectronic device. The optoelectronic device includes a transfer die, also known as a III-V coupon or device coupon, which has been bonded to the bed of a cavity (SOI Cavity) formed in a silicon-on-insulator layer. The III-V coupon includes a III-V waveguide, and is formed of one or more III-V based semiconductors. The III-V coupon includes an alignment mark. In this example, the alignment mark comprises three projecting lines, two of which extend in a first direction and one of which extends in a second direction transversal (in this example perpendicular) to the second direction. The III-V coupon includes a metal patch which aids in providing optical contrast. The metal patch can be on top of the alignment mark, and the alignment mark may include an etched trench which makes the projecting lines protrude from the trench floor and is covered by the metal patch. The alignment mark is around 35 μm wide (as measured in the first direction) and 130 μm long (as measured in the second direction). The alignment mark, at its closest, is around 11.25 μm from the III-V waveguide. It is also, as its closest, 36 μm from the edge of the III-V coupon which is closest to the SOI chip.

There is an interface between the III-V waveguide and a SOI waveguide, the SOI waveguide being located in the SOI chip. The interface is angled at an angle of around 80°, and the gap between the two interfaces, at its closest is around 1 μm. This gap, in this example, is filled with a dielectric fill (for example a benzocyclobutene resin). The SOI waveguide extends into the cavity by a distance which is at most 22.5 μm. The SOI chip also includes an alignment mark. In this example, the alignment mark comprises three projecting lines, two of which extend in a first direction and one of which extends in a second direction transversal (in this example perpendicular) to the first direction. The SOI chip includes a metal patch which aids in providing optical contrast. The metal patch can be on top of the alignment mark, and the alignment mark may include an etched trench which makes the projecting lines protrude from the trench floor and is covered by the metal patch. The alignment mark is located, at its closest, around 11.25 μm from the SOI waveguide. The dimensions of the alignment mark on the SOI chip are, in this example, the same as the dimensions of the alignment mark on the III-V coupon. The alignment mark on the SOI chip is located, at its closest, around 2.5 μm from the edge of the cavity closest to the III-V coupon. The alignment mark on the SOI chip is located, at its closest, around 55 μm from the alignment mark on the III-V coupon.

FIG. 31 shows in more detail the alignment mark(s) of the transfer die. The alignment mark shown in FIGS. 31 and 30 includes three lines, two which extend in the first direction and are around 30 μm long as measured in the first direction, and one which extends in the second direction and is around 35 μm long as measured in the second direction. All of the lines are around 15 μm wide. The two 30 μm long lines are spaced from respective first ends of the metal patch by a distance of around 15 μm, and from respective second ends of the metal patch by a distance of around 2.5 μm. The one 35 μm long line is spaced from the second ends of the metal patch by a distance of around 10 μm.

FIG. 32 shows in more detail the coupling interface of FIG. 30. The III-V waveguide includes a T-bar facet, where the bar of the T is around 2 μm deep (as measured as a depth from the facet itself, as shown in FIG. 32). The III-V waveguide is around 2.5 μm wide, as measured in a direction perpendicular to the guiding direction of the waveguide. The III-V waveguide includes a metal contact along an upper surface thereof, which may be used as the p-electrode and so referred to as a p-metal contact. The SOI waveguide includes a T-bar facet, where the bar of the T is around 1 μm deep (as measured as a depth from the facet itself, as shown in FIG. 32). The SOI waveguide includes a ridge or rib which is around 2.6 μm wide, and a slab with edges which is around 28.6 μm wide. The gap between the SOI waveguide T-bar facet and the III-V T-bar facet is around 1 μm. The T-bar facets are at an angle of around 80° relative to a guiding direction of the III-V waveguide and/or the SOI waveguide.

FIG. 33 shows a further variation of an optoelectronic device. It shares many features with the examples discussed above (for example, the III-V and SOI waveguides are identical although they need not necessarily be so). In this example, the III-V coupon includes two alignment marks which are generally square in shape. The III-V coupon includes two metal patches, which cover the respective alignment marks. In this example, the alignment marks in the III-V coupon are around 110 μm away from corresponding alignment marks in the SOI chip. In this example, there are three alignment marks on the SOI chip. A first which is square in shape, and a second and third which are L-shaped. The square alignment mark is located between the two L-shaped alignment marks, and is spaced from them by around 60.75 μm.

FIG. 34 shows a further variation of an alignment mark used on a transfer die. The alignment mark is generally square in shape, and is for example the type used in the III-V coupon. The alignment mark itself is around 10 μm wide and tall. The trench, which makes the alignment mark protrude from the trench floor, is around 5 μm wide, and the metal patch is around 26 μm wide and tall to cover both the alignment mark and the trench.

FIG. 35 shows a further variation of an alignment mark used on a precursor photonic device. The alignment mark is generally L-shaped, and is for example the type used in the SOI chip shown in FIG. 33. The legs of the alignment mark itself are around 10 μm wide, and so comprises two 20 μm long sides, and four 10 μm long sides. The trench, which surrounds the alignment mark, is around 5 μm wide, and the metal patch is around 5 μm wider than the perimeter of the trench and covers both the alignment mark and the trench.

FIG. 36 shows a further variant of an alignment mark used on a precursor photonic device. The alignment mark is generally square in shape, and is for example the type used in the SOI chip. The alignment mark itself is around 10 μm wide and tall. The trench, which surrounds the alignment mark, is around 5 μm wide, and the metal patch is around 30 μm wide and tall to cover both the alignment mark and the trench.

FIG. 37 shows in more detail the coupling interface of FIG. 33. It is very similar to the coupling interface shown in FIG. 32. It shows that the III-V waveguide includes a T-bar with a width of around 2 μm and which extends at an angle of around 80° relative to the guiding direction of the III-V waveguide. The III-V waveguide is around 2.5 μm wide, and includes a metal contact (e.g. a p-metal contact) located on an upper surface thereof. The SOI waveguide is around 2.6 μm and includes a T-bar facet which is 1 μm wide and extends at the same 80° angle as the III-V T-bar facet. The gap between the III-V and SOI waveguides s around 1 μm. One or more trenches are etched into the silicon device layer to form the SOI waveguide.

FIG. 38 shows a further variant of an optoelectronic device. As before, it includes a III-V coupon and an SOI chip. However in this example there is a first and second SOI waveguide and the III-V waveguide is curved and provides a first and a second facet. The first facet of the III-V waveguide couples to the first SOI waveguide and the second facet of the III-V waveguide couples to the second SOI waveguide. Both facets are located on a same lateral side of the III-V coupon. The III-V coupon includes first and second electrodes, and the III-V coupon may be operable as a modulator (e.g. an electro-absorption modulator) through application of a bias to the first and second electrodes. As before, the III-V coupon includes alignment marks of the type discussed previously (e.g. two square alignment marks). The SOI chip includes three alignment marks, two L-shaped and one square. The distance between the square alignment marks on the III-V coupon and the L-shaped alignment marks on the SOI chip is around 110 μm.

FIGS. 39A-39D show variations of alignment marks. These include a circle as shown in FIG. 39A; a ring as shown in FIG. 39B; a partial circle, e.g. a half circle, as shown in FIG. 39C; and a partial ring, e.g. a half ring, as shown in FIG. 39D. The alignment marks can also be T shaped. Examples of transfer die and precursor devices herein can use any one, or any combination, of these alignment marks.

As has been discussed previously, the alignment marks are fabricated at the same time as the corresponding waveguide. For example, the alignment marks in the III-V coupon are fabricated at the same time as, or in the same mask layer of, the III-V waveguide. Similarly, the alignment marks in the SOI chip are fabricated at the same time as, or in the same mask layer of, the SOI waveguide(s).

The alignment marks disclosed herein are designed for optical waveguide alignment between the III-V coupon waveguides and the SOI waveguides so as to achieve high optical coupling efficiency. On the III-V coupon side, in some examples the alignment marks on the top surface and near the III-V waveguide T-bar facet which has an angle ranging from 80° to 90°. On the SOI chip side, in some examples the alignment marks are on the top surface and near the SOI waveguide T-bar facet which has an angle ranging from 80° to 90°.

When the SOI waveguide and III-V waveguide are aligned, the distance between the alignment marks of the III-V coupon and the alignment marks of the SOI chip in some examples is more than 50 μm and less than 1000 μm. In some examples, there is a metal patch on top of the alignment marks for enhancing optical contrast in the alignment process. The metal on the SOI chip can be Ti, TiN, Al, TVAl, Ti/TiN/AI, or TiN; the metal on the III-V coupon side can be Ti/Au or a gold alloy.

The alignment marks can be shapes such as rectangle, square, L, circle, partial circle, ring, and partial ring. Typically the minimum dimension of the alignment marks is 10 μm. Alignment marks can all be one shape, or multiple different shapes. When aligned, the alignment marks in the III-V coupon and the SOI chip can be either in a line or off a line. When aligned, the alignment marks themselves in either or both of the III-V coupon or SOI chip are symmetrical to the same axis or symmetrical to different axes (i.e. the SOI alignment marks are symmetrical to one axis but the III-V alignment marks are symmetrical to another axis). The alignment marks themselves in either or both of the III-V coupon or SOI chip may be symmetrical. The alignment marks themselves in either or both of the III-V coupon or SOI chip may be aligned in one line or in different multiple different lines.

The SOI waveguide T-bar facet which aligns with the III-V T-bar facet projects into the cavity with a length of around 22.5 μm (but may extend at least 10 μm and no more than 50 μm), and a width of around 28.6 μm (but may be at least 15 μm and no more than 50 μm) with the angle or projection of between 0° and 10° and with or without two rounded corners.

In some examples there are antireflective coating layers on the III-V coupon facet. In some examples these layers include: a 20 nm SiO₂ layer, a 170 nm Si₃N₄ layer, and a 100 nm SiO₂ layer. There is a layer of antireflective coating on the SOI facet which is a 170 nm Si₃N₄ layer. When aligned, there is a filling material which fills the gap between the III-V coupon facet and the SOI facet. In some examples this is a dielectric, for example a benzocyclobutene resin.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

LIST OF FEATURES

-   101 Fine alignment mark(s) on transfer die -   102 Fine alignment mark(s) on precursor device -   103 Coarse alignment mark(s) on transfer die -   104 Coarse alignment mark(s) on precursor device -   200 Precursor photonic device -   201 Input ridge or rib waveguide -   202 Upper surface of device -   203 Cavity -   204 Output ridge or rib waveguide -   205 Alignment marks -   206 Anti-reflective coating -   207 Passivation layer -   208, 716 Upper cladding layer -   209 Lower cladding layer -   210 Substrate -   301 Device layer -   302 Buried oxide layer -   303 Substrate -   304 Cavity patterning layer -   305, 501, 708, Photoresist -   908, 1101, 1301 -   306 Upper surface of device layer -   307 Alignment marks in photoresist -   601 Region outside of III-V device -   602 Transparent region -   604 Si alignment marks -   701 III-V semiconductor stack -   702 P doped InGaAs layer -   703 P doped InP layer -   704 AlInGaAs multiple quantum well region -   705 N doped InP layer -   706 InGaAs sacrificial layer -   707 InP substrate -   709 Mask -   710, 712, 1102, 1103 Alignment area -   711 N contact area -   713 Alignment marks in photoresist -   714 Waveguide -   715, 1115 Alignment marks -   717, 718 Via for electrode connection -   719, 720 Electrodes -   721, 906, 1112 Lifting photoresist -   722 Tether -   902, 1104 Chip boundary area -   904 Alignment marks -   1105 Isolation area -   1106 Silicon nitride layer -   1107 BCB (Benzocyclobutene) fill -   1108 Via for electrode connection -   1109, 1110 Traces for electrodes -   1111 Hard mask -   1113 P contact area -   1302 Alignment marks -   1303 Alignment mark trench -   1701 Facet filling -   2301 Fine alignment mark -   2302 Coarse alignment mark -   2700 Alignment token die -   2800 III-V transfer wafer -   2802 Device die -   2900 Platform wafer -   2902 Precursor photonic device 

1. A method of transfer printing, comprising: providing a precursor photonic device, comprising a substrate and a bonding region, wherein the precursor photonic device includes one or more alignment marks located in or adjacent to the bonding region; providing a transfer die, said transfer die including one or more alignment marks; aligning the one or more alignment marks of the precursor photonic device with the one or more alignment marks of the transfer die; and bonding at least a part of the transfer die to the bonding region.
 2. The method of claim 1, wherein the precursor photonic device, or the transfer die, or both, include one or more metal patches covering the respective alignment marks.
 3. The method of claim 2, wherein the at least one or more alignment marks located in or adjacent to the bonding region or the at least one of the one or more alignment marks included in the transfer die, or both, include one or more trenches etched to surround the alignment marks and covered by the metal patch.
 4. The method of any preceding claim, wherein the precursor photonic device includes a first waveguide and the transfer die includes a second waveguide, and, once bonded, an interface between the first waveguide and the second waveguide is angled relative to a guiding direction of the first waveguide or second waveguide.
 5. The method of claim 4, wherein the first waveguide, the second waveguide, or both, include a T-bar facet.
 6. The method of any preceding claim, wherein at least one of the one or more alignment marks located in or adjacent to the bonding region or at least one of the one or more alignment marks included in the transfer die, or both, are formed in one of the following shapes: a rectangle, a square, an L-shape, a circle, a ring, a partial circle, and a partial ring.
 7. The method of any preceding claim, wherein the precursor photonic device includes one more alignment marks located adjacent to the bonding region, and once bonded, a closest distance between one of the one or more alignment marks adjacent to the bonding region and a corresponding one of the one or more alignment marks included in the transfer die is at least 50 μm and no more than 1000 μm.
 8. The method of any preceding claim, wherein the or each alignment mark located in or adjacent to the bonding region is symmetrical as compared to a corresponding alignment mark included in the transfer die.
 9. The method of any preceding claim, wherein the precursor photonic device is as set out in any of claims 22-33.
 10. The method of any preceding claim, wherein the transfer die is as set out in any of claims 21-32.
 11. The method of any preceding claim, further comprising a step of filling a facet between the precursor photonic device and the transfer die.
 12. The method of claim 11, wherein the filling material used to fill the facet is silicon nitride, amorphous silicon, a dielectric, or a benzocyclobutene resin.
 13. The method of any preceding claim, further comprising one or more steps of: plasma treating the precursor photonic device and/or the transfer die; dipping the precursor photonic device in water; drying the precursor photonic device; and annealing the transfer die and precursor photonic device.
 14. The method of claim 13, wherein the annealing is performed at a temperature of at least 250° C. and no more than 350° C. for a time of at least 20 minutes and no more than 400 minutes.
 15. The method of claim 14, wherein the annealing is performed in an inert gas atmosphere, such as nitrogen atmosphere or argon atmosphere.
 16. An optoelectronic device, produced using the method of any of claims 1-15.
 17. An optoelectronic device, comprising: a silicon-on-insulator wafer, having a cavity; and a III-V semiconductor based photonic device, located within and bonded to the cavity; wherein the III-V semiconductor based photonic device includes one or more alignment marks, which align with corresponding alignment mark(s) on the silicon-on-insulator wafer.
 18. The optoelectronic device of claim 17, wherein the one or more alignment marks are located in an optically transparent region of the III-V semiconductor based photonic device.
 19. The optoelectronic device of claim 17, wherein the one or more alignment marks are voids in the III-V semiconductor based photonic device that extend entirely through the III-V semiconductor based photonic device.
 20. The optoelectronic device of any of claims 17 to 19, further comprising an input and/or output waveguide, the waveguide(s) being provided in the silicon-on-insulator wafer and optically coupled to the III-V semiconductor based photonic device.
 21. An optoelectronic device comprising: the precursor photonic device of any of claims 22-33, bonded to the transfer die of any of claims 34-46.
 22. A precursor photonic device, comprising: a substrate; a bonding region, for receiving and bonding to a transfer die; and one or more alignment marks, for use in transfer printing, said alignment marks being located in or adjacent to the bonding region.
 23. The precursor photonic device of claim 22, wherein the bonding region is in a cavity, provided in the substrate.
 24. The precursor photonic device of either of claim 22 or 23, further comprising an input waveguide, wherein the alignment marks are configured to align a photonic device, located on the transfer die, relative to the input waveguide.
 25. The precursor photonic device of any of claims 22-24, wherein the one or more alignment marks allow for alignment in at least two non-parallel directions.
 26. The precursor photonic device of any of claims 22-25, wherein the one or more alignment marks are provided as one or more etched regions and/or as one or more patterned metal surfaces.
 27. The precursor photonic device of claim 26, wherein the etched regions have a depth of at least 100 nm and no more than 3000 nm.
 28. The precursor photonic device of any of claims 22-27, including one or more coarse alignment marks, and one or more fine alignment marks.
 29. The precursor photonic device of claim 28, wherein the one or more coarse alignment mark(s) project in at least two non-parallel directions
 30. The precursor photonic device of either claim 28 or claim 29, wherein the one or more coarse alignment mark(s) are shaped as any one or more of: an arrow, a cross, a T shape, and an L shape.
 31. The precursor photonic device of any of claims 28-30, wherein there are two or more fine alignment marks which respectively project in at least two non-parallel directions
 32. The precursor photonic device of any of claims 28-31, wherein the one or more fine alignment mark(s) include Vernier patterns.
 33. The precursor photonic device of any of claims 22-32, wherein the precursor photonic device is a silicon-on-insulator wafer, including either or both of an input waveguide and an output waveguide, each adjacent to the bonding region.
 34. A transfer die comprising: a photonic device, said photonic device having a bonding surface suitable for bonding to a precursor photonic device; wherein the transfer die includes one or more alignment marks, for use in a transfer-print process.
 35. The transfer die of claim 34, wherein the photonic device is a III-V semiconductor device and/or the transfer die includes a sacrificial layer.
 36. The transfer die of either claim 34 or claim 35, wherein the photonic device is a laser, semiconductor optical amplifier, or an electro-absorption modulator.
 37. The transfer die of claim 36, wherein the photonic device is an electro-absorption modulator, wherein said electro-absorption modulator comprises an input waveguide and an output waveguide, and wherein both of said input waveguide and said output waveguide comprise a port located on a same side of the transfer die.
 38. The transfer die of any of claims 34-37, wherein the photonic device is formed at least partially from indium phosphide, and/or a sacrificial layer is formed of indium gallium arsenide.
 39. The transfer die of any of claims 34-38, wherein the alignment marks are provided on an optically transparent region of the transfer die.
 40. The transfer die of any of claims 34-38, wherein the alignment marks are voids in the transfer die which extend entirely through the transfer die.
 41. The transfer die of any of claims 34-40, formed on an indium phosphide substrate.
 42. The transfer die of any of claims 34-41, wherein the photonic device includes one or more coarse alignment marks and one or more fine alignment marks.
 43. The transfer die of claim 42, wherein the one or more coarse alignment mark(s) project in at least two non-parallel directions.
 44. The transfer die of either of claim 42 or 43, wherein the one or more coarse alignment mark(s) are shaped as any one or more of: an arrow, a cross, a “T” shape, and an “L” shape.
 45. The transfer die of any of claims 42-44, wherein there are two or more fine alignment marks which respectively project in at least two non-parallel directions.
 46. The transfer die of any of claims 42-45, wherein the one or more fine alignment mark(s) include Vernier patterns.
 47. A platform wafer, suitable for use in a transfer printing process, said platform wafer including: one or more alignment chips, said alignment chips including one or more alignment marks; and one or more precursor photonic device(s).
 48. The platform wafer of claim 37, wherein the precursor photonic device is as set out in any of claims 22-33.
 49. A transfer wafer, suitable for use in a transfer printing process, said wafer including: one or more alignment chips, said alignment chips including one or more alignment marks; and one or more device chips.
 50. The transfer wafer of claim 49, wherein the transfer wafer includes one or more transfer dies as set out in any of claims 34-46.
 51. A method of preparing a precursor photonic device, comprising the steps of: providing a wafer, comprising a substrate and a device layer; and etching one or more alignment marks into the wafer.
 52. The method of claim 51, further comprising: etching a cavity into the wafer, said cavity extending from an uppermost surface of the device layer to at least an uppermost surface of the substrate; and etching the one or more alignment marks into the substrate.
 53. The method of either of claim 51 or 52, further comprising a step of etching at least one of an input waveguide and an output waveguide, said input waveguide and/or output waveguide having a surface adjacent to the cavity.
 54. The method of claim 53, wherein the step of etching the one or more alignment marks is performed at the same time as etching the input waveguide and/or output waveguide.
 55. The method of claim 54, wherein the step of etching the one or more alignment marks and the input waveguide and/or output waveguide comprises the sub-steps of: (a) providing a photoresist over an upper surface of the precursor photonic device; (b) patterning the photoresist to provide one or more exposed regions; and (c) etching the exposed regions.
 56. The method of any of claims 51-55, further comprising a step of depositing an antireflective coating, preferably formed of silicon nitride, along either or both of: one or more sidewalls; and/or a bed of the cavity.
 57. The method of claim 56, further comprising a step of removing at least the antireflective coating present adjacent to the alignment marks.
 58. The method of any of claims 51-57, further comprising a step of depositing a top cladding layer over the exposed upper surface of the precursor photonic device, after the step of etching the one or more alignment marks.
 59. The method of claim 58, comprising a step of removing portions of the top cladding layer which are within the cavity.
 60. A method of forming a transfer die, comprising the steps of: providing a multi-layered structure, said multi-layered structure including at least a sacrificial layer and one optically active layer; and etching one or more alignment marks into a part of the multi-layered structure.
 61. The method of claim 60, wherein the step of etching one or more alignment marks is performed concurrently with a step of etching one or more device structures into the multi-layered structure.
 62. The method of claim 60, further includes a step of depositing a stress compensation layer.
 63. The method of any of claims 60-62, wherein etching one or more alignment marks into a part of the multi-layered structure includes etching a region of the transfer die such that it is optically transparent.
 64. The method of any of claims 60-63, wherein the alignment marks are etched entirely through the multi-layered structure. 